1. Field of the Invention
The invention relates to narrowband filters and, in particular, to a means for providing high speed filtered shuttering for large aperture optics.
2. Background
There is a need for an inexpensive, fast, versatile and reliable shutter mechanism that can provide blur-free imagery of rapidly changing scenes across the visible and infrared portion of the spectrum for large aperture optics, including state-of-the-art detection and image systems with large optical apertures. This shuttering action is necessary in many instances to increase signal-to-background ratio of pulsating irradiance levels, obtain blur-free imagery of high-speed, low-retention imagery, provide single-event observations and produce flexible intensity control on incoming radiation. Systems with large optical apertures like telescopes present particular problems because performing shuttering outside the optics requires large filters which are difficult to construct and operate.
It is known in the art that it is advantageous to provide the shuttering for such systems between the optics and the focal plane in order to reduce the shutter size and complexity. However, none of the prior art mechanisms can simply satisfy this requirement for repetitive, high-speed shuttering between the optics and the focal plane. The most common prior art mechanism used for this purpose is a rotating chopper wheel that is installed close to the focal plane and "sweeps" or "wipes" across the image plane. The chopper wheel has several disadvantages. One disadvantage is that a fully open aperture occurs at only one instant of time. Since the chopper wheel is continually masking different segments of the focal plane, video signals from rapidly moving targets and radiant intensity levels vary with both target position and movement on the image plane during open-aperture sweep times. Another important disadvantage of the chopper wheel is that it is difficult to construct a chopper wheel which produces very rapid shuttering, e.g. sub-millisecond shuttering, for large focal planes.
Sub-millisecond shuttering can be achieved by the use of long focal-length optics and mechanical chopping where image irradiance converges to a point in front of the focal plane. However, this approach is not compatible with short focal-length, high-speed optics because the location, alignment, and size of the chopper aperture become exceedingly critical. Any slight change in target position during open-aperture period results in a change in image signal irradiance, which could vary on a frame-by-frame basis.
Conventional shutter systems have been adapted to large aperture optics using additional lens systems to decrease image plane size for faster shuttering speeds and then to redefine the original imagery onto the focal plane. For example, a typical chopper wheel assembly may have the telescope imagery prefocused and demagnified in the chopper aperture plane and then use subsequent optics to reform the image onto the focal plane. Other interactions of this concept have been devised to setup choppers, rotating mirrors, planner shutter, and vibrating reflectors between optics and focal plane. Unfortunately, these opto-mechanical techniques do not eliminate the sweeping shutter effect and they are not easily applicable to a wide range of optical systems. In addition, the conventional electro-mechanical approaches must be tailored for each specific optical application. Furthermore, it becomes difficult and expensive to implement at infrared wavelengths where diffraction effects may result in deteriorated optical resolution.
Other conventional shutter systems cannot be relocated to the space between the optics and the focal plane. In a related invention entitled "A Spectrally Selective Shutter Mechanism", disclosed in August 1984 Proceedings of S.P.I.E., the relative movement of two narrowband spectral filters is used to limit transmitted irradiance. However, at least one of those filters must be installed in front of the optical system and the diameter of that filter must be larger than the diameter of the aperture of the optics. For apertures greater than about 4 inches, the size of the outside filter and the size and speed of the motor to rotate the filter make the use of such a mechanism impractical because it becomes progressively more difficult to fabricate high quality, large area spectral filters and to design the required fast, high torque tilting apparatus.
A final, conventional shuttering mechanism which is placed between the optics and the focal plane is the microchannel plate. This mechanism consists of microscopic metal capillaries connected to a phosphorous plate. The capillaries are connected to a high voltage and transfer the incoming irradiance to an electron field which amplifies the signal onto the phosphorous plate. Shuttering is performed by controlling the application of the voltage. Microchannel plates have several disadvantages. One is that operation is limited to a specific wavelength region. Another is that use of microchannel plates is accompanied by unwanted noise.
It is therefore an object of this invention to provide a fast and reliable shutter mechanism which produces distortion free imagery of rapidly changing scenes for large aperture optics.
It is also an object of this invention to provide a shutter mechanism which is capable of performing in the sub-millisecond range.
It is a further object of this invention to provide a versatile shutter that can operate in the visible and infrared spectral regions.
It is a further object of this invention to provide a shutter mechanism capable of performing at very low f-numbers without a complex arrangements of optical elements.
Yet another object of this invention is to furnish a mechanism for tailoring spectral characteristics of existing narrow-band filters.